Wet etching is a commonly used method in the manufacturing of integrated circuits. For example, in damascene processes, titanium nitride is commonly used to form hard masks, which are used to define the patterns of trenches and via openings, in which copper is filled to form metal lines and vias. Hence, hard masks need to be patterned first, so that their patterns may be transferred to the underlying dielectric layers. Traditionally, after the formation of the metal lines and vias, the hard masks are removed by a Chemical Mechanical Polish (CMP) process.
The removal of titanium nitride may be performed using a wet etch (wet clean) process, during which a chemical solution is used to etch the titanium nitride (TiN). Conventionally, there were two types of wet-clean circulation system. The first is known as the drain-mode system. In the drain-mode system, the wet-clean chemical is pumped into a tank for storage. From the storage, the wet-clean chemical is passed through a particle-filter for filtering large particles. The wet-clean chemical is then heated, and then sent to a process chamber, in which wafers are sprayed with the wet-clean chemical in order to remove the hard mask. The drain-mode wet-clean circulation system results in more waste since the wet-clean chemical is not reused. The manufacturing cost is also high due to the cost of the wet-clean chemical.
The second type of wet-clean circulation system is known as the circulation-mode system. In the circulation-mode system, the wet-clean chemical is pumped into a tank for storage. From the storage, the wet-clean chemical is passed through a particle-filter for filtering large particles. The wet-clean chemical is then heated, and then conducted to a process chamber, in which wafers are sprayed with the wet-clean chemical in order to remove the hard mask. The sprayed wet-clean chemical, which has been used, is then collected, and pumped back to the storage, so that it can be filtered, heated, and sent back to the process chamber again. By using the circulation-mode wet-clean circulation system, the waste of the wet-clean chemical is reduced. However, the dissolved titanium ions remain in the wet-clean chemical, and remain in the circulation system and the pipelines. With the increase in the amount of titanium ions in the wet-clean chemical, the clean efficiency and the TiN removal rate in the wet clean process are reduced. Furthermore, due to the variation in the amount of titanium ions in the wet-clean chemical, the efficiency in the removal of the titanium nitride has variations accordingly, causing variations in the resulting hard mask clean and removal process.